The present invention relates to fuel cells, and more particularly, but not exclusively relates to electrochemical fuel cells for which reduction reactions occur at the cathode side using hydrogen peroxide. This reduction process, when combined with the oxidization reaction at the anode side, generates electrical energy.
Aluminum-hydrogen peroxide (Al/H2O2) semi fuel cells have been studied for underwater propulsion. The existing problem with the Al/H2O2 semi fuel cell is that the energy density is still lower than desired for many applications—particularly space propulsion implementations. While hydrogen peroxide H2O2 is used indirectly to generate oxygen gas for utilization at the cathode, there are significant difficulties from doing so. For example, in a fuel cell using air or oxygen on the cathode side, the oxygen joins the reduction reaction in a gaseous form. Because the mass density achievable in this gas phase is ordinarily a thousand times less than that available in a liquid phase, the area current density is at least 100 times less from this limiting factor alone. To address this issue, ordinary fuel cells typically use a compressor to pressurize the air/O2 to a few Bars. Even so, the current density is still at least 30 times less than the liquid phase counterpart. The additional weight and energy requirement of the pressurizing system also represent performance penalties.
Furthermore, the mass transport of the reactants in such fuel cells is a two-phase process. In a proton exchange membrane fuel cell in particular, the two-phase transport of reactant and product species can be a limiting phenomenon of fuel cell operation. Particularly, at high current densities, transport of oxygen to the catalyst affects the oxygen reduction reaction rate in the cathode. Furthermore, the water generated in cathode reaction condenses when water vapor exceeds the saturation pressure, and blocks the open pores of the gas diffusion layer, further limiting reactant transport.
The slow kinetics of oxygen reduction has also been identified as a factor limiting the current density and the overall energy conversion efficiency of an oxygen fuel cell system. The oxygen reduction reaction at the cathode is written as: O2+4 H++4 e→2 H2O. This reaction involves four electrons simultaneously, and therefore has a low probability of occurrence. Alternatively the poor kinetics of the oxygen reduction reaction can also be attributed to the low exchange current density of the oxygen reduction reaction. The high cathodic overpotential loss of 220 mV, at potentials close to the open circuit, observed in the current low Pt loading electrocatalyst, is due to a mixed potential that is set up at the oxygen electrode. This mixed potential is from a combination of slow O2-reduction kinetics and competing anodic processes such as Pt-oxide formation and/or impurity oxidation. Further, the low exchange current density of the O2-reduction reaction results in a semi-exponential, Tafel-like behavior—indicating that the reaction is activation controlled over a range of three orders of magnitude in current density. It has been found that the exchange current density of O2-reduction is 6 orders of magnitude lower than that of H2-oxidation reaction. Thus, there are numerous limitations associated with oxygen gas reduction at a fuel cell cathode.
Accordingly, there is a need for further contributions in this area of technology.